1. Field of the Invention
The present invention relates to an improved and novel lighting circuit for a discharge lamp, which detects the short-circuit state between the ground and AC output terminals where a discharge lamp is connected and stops the operation of the lighting circuit, thereby protecting the lighting circuit and preventing an electric shock.
2. Description of the Related Art
Recently, compact metal halide lamps are receiving greater attention as a light source to take the place of an incandescent lamp. A known lighting circuit for a vehicular metal halide lamp is designed to use a DC power supply for a power source in such a way that a DC input voltage after being boosted by a DC booster circuit is converted to an AC voltage with a sine waveform or a square waveform which is in turn applied to the metal halide lamp.
FIG. 8 shows one example of such a lighting circuit shown by a. A DC voltage from a battery b is input to a DC booster circuit e via a lighting switch c and a protection circuit d and is then converted to an AC voltage by a DC-AC converter f.
An igniter circuit g, provided at the subsequent stage of the DC-AC converter f, generates a high-voltage pulse when a metal halide lamp h is activated, and super-imposes this pulse on the output voltage of the DC-AC converter f. The igniter circuit g then applies the resultant voltage to the metal halide lamp h. The metal halide lamp h is connected to AC output terminals i and i'.
A short-circuit detector j detects the output voltage of the DC-AC converter f to detect the occurrence of a short-circuit state due to the disconnection of a connector, water penetration or the like. This detector j sends a detection signal to the protection circuit d to stop the operation of the lighting circuit to cut off the power supply to the metal halide lamp g.
In detecting short-circuiting, the following three short-circuit states should be considered.
(1) Short-circuit between the AC output terminals i and i'. PA1 (2) Short-circuit between the AC output terminal i and ground. PA1 (3) Short-circuit between the AC output terminal i' and ground.
FIG. 9 exemplifies the structure of the short-circuit detector j which has three comparators.
A comparator k detects the event (1) or (2). This comparator k has one input terminal connected to the line that connects one output terminal of the DC-AC converter f to one AC output terminal i, with a reference voltage from a constant voltage supply l being supplied to the other input terminal. The comparator k outputs a high (H) signal when the potential at the AC output terminal i becomes equal to or lower than the reference voltage.
A comparator m detects the event (1) or (3). This comparator m has one input terminal connected to the line that connects one output terminal of the DC-AC converter f to one AC output terminal i', with a reference voltage from a constant voltage supply n being supplied to the other input terminal. The comparator m outputs an H-level signal when the potential at the AC output terminal i' becomes equal to or lower than the reference voltage.
A comparator o has one input terminal connected to one output terminal of the DC-AC converter f and the other input terminal connected to the other output terminal of the DC-AC converter f. The comparator o compares the potentials at the AC output terminals i and i' with each other and outputs an H-level signal as the detection output for the event (2).
The output terminals of those comparators k, m and o are connected via diodes in an OR form to be connected to the input terminal of a hold circuit p in the protection circuit d.
The short-circuit detector j detects the event (1) when the outputs of the comparators k and m both have an H level, detects the event (2) when the output of the comparator o has an H level, and detects the event (3) when the output of the comparator m has an H level. Those detection signals work on the protection circuit d via the hold circuit p to inhibit the operation of a lighting circuit a. This inhibited state is maintained until a lighting switch c is activated again.
The above conventional lighting circuit requires three comparators to detect short-circuiting, and particularly uses two comparators to respectively detect the short-circuit states (2) and (3). This design has a complicated circuit structure and suffers an increased number of elements.